In optical communications, optical signals carry information. For example, a transmitter (e.g., a laser or laser diode) in an optical or optoelectronic transceiver converts one or more electrical signals into optical signals, and a receiver (e.g., a photodiode) in an optical or optoelectronic transceiver converts one or more optical signals into electrical signals. One objective of optical communication research and development is to increase and/or maximize bandwidth (e.g., the amount of information transmitted) to the greatest extent possible. Another objective is to communicate and receive the information as reliably as possible.
Typically, optical receivers for such optical signals include a photodiode (e.g., an avalanche photodiode, or APD). There are existing methods for protecting APDs from damage. According to theory, the primary cause of APD damage in practical use includes damage from high voltage (e.g., high Vapd) and damage from high current (e.g., high Iapd). Setting a control voltage Vopt to an APD at the optimal sensitivity point can ensure that the APD operational voltage is small enough to avoid damage to the APD. High current in the APD (e.g., Iapd>2 mA) can cause permanent damage to the APD. As photodiodes generally have a quick response to light, current-limiting protection downstream from the photodiode cannot limit instant light-induced current, and current-limiting resistors generally limit changes in light-induced current by regulating Vapd.
FIG. 1 is a diagram showing a conventional circuit 10 for protecting an APD 22 in a receiver optical subassembly (ROSA) 20 in an optoelectronic receiver. In the circuit 10 as shown in FIG. 1, the microprocessor MCU 30 controls the maximum voltage to the APD 22 using a feedback signal 54 from a boost control circuit 40 and a current minor 50. A high voltage or current can be provided to the ROSA 20 from a first output 52 of the current mirror 50. The output 52 supplies a voltage to the APD 22 after filtering by two decoupling capacitors C1 and C2. As a result, the APD 22 receives a sufficient voltage to cause an avalanche and/or multiplier effect. The APD 22 then converts optical signals into electrical signals (which are output as the APD current Iapd), and a transimpedance amplifier (TIA) 60 converts the single-ended APD current Iapd into a differential signal and shapes the differential signal into a data signal Data+/Data− that is received by a limiting amplifier 70. If the amplitude of the data signal Data+/Data− has an insufficient magnitude, the differential limiting amplifier 60 determines that the data signal Data+/Data− is lost, and it outputs an LOS (loss-of-signal) signal 75 having a state representing this condition. Another output 54 of the current minor 50 proportional to the current on the first output 52 generates a current sampling voltage across a sampling resistor R1 that is input at a receiver power sample input (e.g., RX power pin) to the MCU 30, and the receiver power is sampled by an analog-to-digital converter (ADC) in the MCU 30 and monitored by the MCU 30.
For optimum sensitivity, the MCU 30 controls the boost control circuit 40 such that the bias current 52 to the receiver APD 22 remains in an avalanche mode at or near a point where a multiplication factor of the APD 22 converts a relatively small light input to a relatively large current. However, when the bias input to the APD 22 is too large, the current flowing through the APD 22 is too large, and the APD 22 can be damaged.
FIG. 2 is a diagram showing an improvement to the circuit of FIG. 1, in which the current 52 at the first output terminal of the current mirror 50 is connected to a second resistor R2 having a resistance of, e.g., 10-100 kΩ. The resistance is relatively large when the strength of the optical signal received at the APD 22 increases. The resulting voltage drop across the second resistor R2 reduces the voltage to the APD 22, reducing the APD multiplication factor and the current Iapd, thereby protecting the APD 22.
FIG. 3 is a diagram showing exemplary communication paths or links in a system 100 for use in an optical network. For clarity, only the transmission (outbound) paths are shown. The system 100 includes a network-side optical or optoelectronic dual transceiver 110, a main host-side optical or optoelectronic transceiver 120, a reserved or duplicate host-side optical or optoelectronic transceiver 130, and an optical multiplexer/demultiplexer 140. The system 100 may operate or function as an optical switch, repeater and/or regenerator.
The network-side transceiver 110 receives first and second incoming optical signals 112 and 114 at first and second incoming data ports TI1 and T12. The network-side transceiver 110 transmits the first and second optical signals 112 and 114 through ports TO/11 and TO21 to the main host-side transceiver 120 and duplicate optical signals 112′ and 114′ through ports TO12 and TO22 to the reserved or duplicate host-side transceiver 130. The main and reserved/duplicate host-side transceivers 120 and 130 respectively transmit optical signals 122 and 132 through respective ports OUT and OUT' to the optical multiplexer/demultiplexer 140. The optical signal 122 may be or comprise either the first or second optical signal 112 or 114, a combined version of the first and second incoming optical signals 112 and 114 (e.g., by wavelength division multiplexing), or a reframed or reformatted version of the first and/or second incoming optical signals 112 and 114 (e.g., having different overhead data/information, transmitted at a different rate and/or frequency, etc.). The optical signal 132 may be identical or substantially identical to the optical signal 122.
During normal operation, the links from ports TO11 and TO21 in the network-side transceiver 110 to the receiver ports RX1 and RX2 in the host-side transceiver 120 are operational, and the optical multiplexer/demultiplexer 140 outputs the optical signal 122 from the host-side transceiver 120 as the data signal OUT to the host. However, if one of the links from the network-side transceiver 110 to the host-side transceiver 120 fails, a redundant link from port TO12 or TO22 in the network-side transceiver 110 to the port RX1′ or RX2′ in the reserved or duplicate host-side transceiver 130 can be established, and the optical multiplexer/demultiplexer 140 can be instructed to output signal 132 from the reserved or duplicate host-side transceiver 130 in place of the signal transmitted over the failed link.
In one common implementation, the new link between the other transmitter and the receiver must be established within a predetermined time period (e.g., 50 ms), for example to avoid loss of the link and/or of significant amounts of data transmitted over the link. However, when switching from one transmitter to another, the power or signal strength of the optical signal received from the new transmitter may be significantly higher than the optical signal from the old transmitter. When the power or signal strength of the new optical signal exceeds a maximum threshold, the current in the APD of the new receiver may exceed a limit above which the APD may be damaged. The circuit 10 of FIG. 1 is generally unable to protect the APD against such excessive currents after switching to a new transmitter. The circuit 10′ of FIG. 2 can protect the APD against changes in the optical power during periods of continuous operation, but due to a relatively slow response time, it cannot protect the APD against excessive currents after switching to a new transmitter.
This “Discussion of the Background” section is provided for background information only. The statements in this “Discussion of the Background” are not an admission that the subject matter disclosed in this “Discussion of the Background” section constitutes prior art to the present disclosure, and no part of this “Discussion of the Background” section may be used as an admission that any part of this application, including this “Discussion of the Background” section, constitutes prior art to the present disclosure.